1. Field of the Invention
The invention concerns a method of operating a digital transmission link, of the radio or cable bus type, time-shared by a plurality of units under the control of one of the units for exchanging communication signals. It also concerns units equipped to implement the method.
2. Description of the Prior Art
The use of a transmission link by a plurality of units to exchange communication signals generally implies sharing the transmission capacity available by virtue of the link to give the units communication facilities optimally matched to their requirements.
In the case of a time-shared link, each unit producing communication signals must have temporary use of the link to send and a hardware and/or software structure must therefore be provided for distributing transmission times to the units.
A structure of the above kind in practise implies the generation of clock signals to control sending by the units and it is beneficial to produce synchronization signals to facilitate the sharing of the link by the units, for example to produce such signals periodically.
A frame type temporal organization for operating a link implying periodic production of synchronization signals to which the units can synchronize in order to send and receive data at times determined by the distribution structure relative to the synchronization signals and clock signals is known in itself. The times allocated to a unit for sending can be divided in time within successive time frames during each of which the unit is reserved the same number of successive instants, for example eight such instants, defining a module, the modules used by a unit for sending being temporally disposed in an identical manner in the successive frames that contain them. An organization of the above kind enables synchronous transmission of communication signals, and in particular of PCM encoded speech signals or data, using one or more transmission channels each corresponding to one or more modules identically disposed in successive frames. It is more specifically suited to transmitting information from units with respective regular bit rates or with respective bit rates that can be regularized by buffering before transmission if the link does not provide for real time transmission at a sufficiently high bit rate.
It is therefore difficult to use links of the above kind for bit rates that vary significantly unless the bit rates chosen for the links are high enough to accommodate peaks, which is not necessarily acceptable from the economic point of view.
For various reasons there is not necessarily a good match between the requirements of the units and the means making bit rate available to them at different times.
In another form of organization for transmitting packets of digital data or digitized signals that is known in itself, the instants allocated to a unit for sending can be successive, generally with at least a limitation as to the number of successive instants that can be allocated to the same unit. As in the case of cell type packets, the limitation on the number of successive instants allocated to the same unit can enable asynchronous transmission under specified conditions of signals intended to be transmitted synchronously, such as PCM samples relating to a telephone call. It is also possible to transmit information temporarily arriving at a high bit rate because the number of successive instants that can be obtained by a unit for sending is much greater than that obtained with the organization referred to above and because the flow of information for this unit can resume sooner if the number of units wanting to send on the transmission link is low and their respective bit rate requirements are low. However, there is again not necessarily a good match between the requirements of the units and the means making bit rate available to them at different times. In the present instance, this is in particular a consequence of the fact that the organization governing a link operated in packet or cell mode takes account only of requests submitted by units wanting to send, without actually knowing the nature of the requirements that these requests reflect. In some cases at least, for example in the case of units that can be operated as multimedia terminals, the variations in bit rate and conditions of operation of the same unit can vary greatly depending on the use that is made of them.
The invention therefore proposes a method of operating a link for transmission of digital information time-shared between a plurality of units under the supervision of a control unit that is one of said units assuring arbiter and/or manager functions and controlling transmission of cells between units in frames defined by clock and synchronization signals used by said units in which method said control unit communicates with the other units by means of individually addressed signaling cells for regularly interrogating each of said other units by means of requests relative to its current status and to its communication requirements using said link, and for indicating to it at least one particular frame time slot for its response to the request received and for sending, and each unit having received a signaling cell containing a request from said control unit responds by sending a response signaling cell in a frame time slot as defined by said control unit to specify its status and its requirements.
The invention also proposed a unit for use in a set of units time-sharing a link for transmitting information in digital form under the supervision of one of said units, hereinafter referred to as the control unit, assuring arbiter and/or manager functions and controlling transmissions effected by means of cells between units in cells in the context of the frames defined by clock and synchronization signals processed by said units, including hardware and software means for implementing the above method.
The invention, its features and its advantages are explained in the following description with reference to the figures listed below.